TinEye is an image search utility that analyzes uploaded or linked images to find identical images across the internet. It is surprisingly accurate at identifying the essential parts of an image and matching them to another, which makes it uniquely suited to finding unauthorized use of your photos.
Take a look at this search (it is a picture of earth). Notice how it isn't just finding the same photos over and over again, but also copies of the photo that have been turned into a logo, and even one where it is in the background of someone's presentation.
That means if someone takes your image, turns it into a hideous, stolen, offensive logo for their whatever, TinEye should be able to find it. And while not everyone has issues with their images being stolen, it is always worth a little peace of mind to be able to look and confirm that no one has stolen your image.